Packages and/or packaging materials having a pressure relief valve are known from EP 0144 011 B2, EP 0 559 598 B1, WO 88/07479, DE 25 37 317 A1, EP 0 023 703 B1, EP 0 738 227 A1, DE 44 35 492 A1, EP 0 760 790 B1, WO 2004/048225 A1 and WO 2006/012282 A1. EP 0144 011 B2, EP 0 559 598 B1, WO 88/07479, WO 2004/048225 A1 and WO 2006/012282 A1 all disclose packaging materials wherein a liquid film and spacer means may be provided in the channel region between the first inner film and the second outer film.
It should further be noted that in general flexible packaging materials are provided with an outer oriented film, such as an OPP (oriented polypropylene), OPA (oriented polyamide) or PET (polyethylene terephthalate) as suggested in WO 2004/048225A1 in order to provide the outer film with a print. However, when using such films for the outer film in a packaging material with an integral valve, the opening pressure of the valve is rather high due to the stiffness or comparatively low elasticity of these films.